You are looking at a Transformers Soundwave mp3 player made by Takara and quite possible one of the coolest things I've ever seen. It even plays mp3s in both cassette player mode AND robot mode! It needs a 1GB (a 2GB card won't work) SD card as it has no internal memory, but the card goes in the cassette deck. It is pretty expensive as it's only availabe in Japan and comes in the classic blue version and a white variant version.
As if that wasn't cool enough you can also Frenzy and Rumble head phones!
These are really cool, but looking on eBay the cheapest I saw was $65 which may sound cheap, but the shipping is $65. I saw some guy selling a set of the blue Soundwave, white Soundwave and the Frenzy and Rumble headphones for $254. Holy crap. I bought a scratch ticket today and if I win this is so going to be mine.